Which of the following tools would NOT be suitable for measuring the diameter of a cylinder?

A thickness gauge is primarily designed to measure the thickness of materials rather than the diameter of cylindrical objects. It typically functions by measuring the distance between two surfaces and is more applicable for flat or sheet materials. The other tools listed—vernier calipers, dial indicators, and micrometers—are specifically designed to measure the dimensions of cylindrical objects.

Vernier calipers can measure both external and internal dimensions, and they feature jaws that can grasp cylindrical surfaces effectively. Dial indicators provide precise measurements and can be advantageous for gauging the diameter of a cylinder with added versatility in functionality. Micrometers offer high accuracy in measuring small diameters by using a screw mechanism, which makes them particularly suitable for cylinder measurements. Thus, the thickness gauge is not appropriate for measuring diameter, confirming why it stands out among the options presented.